CSCE3193: Programming Paradigms

Transcription

1 CSCE3193: Programming Paradigms Nilanjan Banerjee University of Arkansas Fayetteville, AR Programming Paradigms 1

2 Java Packages Application programmer interface (API) All classes provided to programmers along with the Java compiler (e.g. Math or MouseEvent) Java expects to find these classes in separate directories or folders The classes stored in each directory form a package The package names are formed by concatenating the directory names starting from a particular root directory

3 Some Predefined Java Packages Package Name java.applet java.awt java.awt.event java.awt.image java.awt.peer java.io java.lang java.net java.util Contents Classes for implementing applets Classes for graphics, windows, and GUI s Classes supporting AWT event handling Classes for image handling Interface definitions s for platform independent graphical user interfaces (GUI s) Classes for input and output Basic language classes like Math (always available in any Java program) Classes for networking Useful auxiliary classes like Date

4 Package Component Names Using a fully qualified component name x = java.lang.math.sqrt(3); Using an import statement // to allow unqualified references to // all package classes import package.name.*; // to allow unqualified references to // a particular package class import package.name.class_name;

5 Import Examples This code java.util.date d = new java.util.date(); java.awt.point p = new java.awt.point(1,2); java.awt.button b = new java.awt.button(); Can be abbreviated import java.util.date; Import java.awt.*; Date d = new Date(); Point p = new Point(1,2); Button b = new Button();

6 Creating Your Own Packages Each package class must be stored in a file in an appropriately named directory The source code file for each package class must contain a package statement as its first noncommented statement package package_name; Several packages can be stored in the same directory Classes in different directories cannot be part of the same package

7 Visibility Rules and Packages Instance variables declared as public or private have the same visibility to classes in other packages Instance variables without explicitly declared visibility have package visibility Instance variables with package visibility are only visible to methods defined in classes belonging to the same package Similarly for static variables, instance methods, and static methods having package visibility Classes not explicitly declared public are not visible outside the package

8 Inheritance Allows programmers to customize a class for a specific purpose, without actually modifying the original class (the superclass) The derived class (subclass) is allowed to add methods or redefine them The subclass can add variables, but cannot redefine them

9 Inheritance Example Class C is a subclass of class B (its superclass) if its declaration has the form class C extends B { } The subclass is a specialization of the superclass The superclass is a generalization of the subclass

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11 Inheritance and Messages When C is a subclass of B C objects can respond to all messages that B objects can respond to In general C objects can be used whenever B objects can be used It is possible that a subclass of B may have methods and variables that have not been defined in B It is the case B objects may not always be used in place of C objects

12 Inheritance Hierarchy A class may have several subclasses and each subclass may have subclasses of its own The collection of all subclasses descended from a common ancestor is called an inheritance hierarchy The classes that appear below a given class in the inheritance hierarchy are its descendaents The classes that appear above a given class in the inheritance hierarchy are its ancestors

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14 Inheritance and Visibility Rules Private variables and methods are not visible to subclasses or clients Public variables and methods are visible to all subclasses and clients Variables and methods with package visibility are only visible to subclasses and clients defined in the same package as the class A variable or method declared with the protected visibility modifier can only be referenced by subclasses of the class and no other classes

15 Overriding vs Overloading A method is overloaded if it has multiple definitions that are distinguished from one another by having different numbers or types of arguments A method is overridden when a subclass gives a different definition of the method with the same number and types of arguments

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17 Constructors The general rule is that when a subclass is created Java will call the superclass constructor first and then call the subclass constructors in the order determined by the inheritance hierarchy If a superclass does not have a default constructor with no arguments, the subclass must explicitly call the superclass constructor with the appropriate arguments

18 Using super( ) Call Constructor The call to super must be the first statement in the subclass constructor Example: class C extends B { public C ( ) { } super( B s constructor arguments );

19 Calling Overridden Superclass Methods from Subclassess The following code generates an infinite loop because tostring( ) is interpreted as this.tostring( ) public void tostring() { String result = tostring(); return (result + : + second); } To make a call tostring in the superclass instead public void tostring() { String result = super.tostring(); return (result + : + second); }

20 Creation of Subclass Instances Assuming that PreciseClock is a subclass of the Clock class, the following is legal Clock dawn; dawn = new PreciseClock(3,45,30); The instance variable dawn will respond to all PreciseClock messages It is not legal to write this since Clock objects cannot respond to all PreciseClock messages PreciseClock dawn; dawn = new Clock(3,40);

21 Static and Dynamic Binding Static Binding Determining which method will be invoked to respond to a message at compile time Dynamic Binding Determining which method will be invoked to respond to a message at run time Required when method definitions are overridden in subclasses, since type of the receiver class may not be known until run time

22 Abstract Classes Abstract classes are only used as super classes Classes are declared as abstract classes only if they will never be instantiated Abstract classes contain usually one or more abstract methods Example: public abstract class Mouse implements Direction { abstract void makemove( ); }

23 Abstract Methods Abstract methods have no body at all and just have their headers declared The only way to use an abstract class is to create a subclass that implements each abstract method Concrete classes are classes that implement each abstract method in their superclasses Example: abstract void makemove( );

24 Interfaces Abstract classes and their extension are related to each other, interfaces neednot You can implement more than one interface but can extend only one abstract class You can implement some methods in abstract classes but not for interfaces With abstract classes you are extending functionality but with interfaces you are just implementing functionality.